Activate Ashnod's Altar by sacrificing Iron League Steed, adding {C}{C}.
When Iron League Steed dies, Nim Deathmantle triggers, causing you to pay {4} to return Iron League Steed from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Iron League Steed enters the battlefield, its fabricate ability triggers, creating a 1/1 Servo artifact creature token.
Activate Ashnod's Altar by sacrificing the Servo, adding {C}{C}.
Repeat.

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite sacrifice triggers

Nim Deathmantle

Ashnod's Altar

Iron League Steed

Activate Krark-Clan Ironworks by sacrificing Iron League Steed, adding {C}{C}.
When Iron League Steed dies, Nim Deathmantle triggers, causing you to pay {4} to return Iron League Steed from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Iron League Steed enters the battlefield, its fabricate ability triggers, creating a 1/1 Servo artifact creature token.
Activate Krark-Clan Ironworks by sacrificing the Servo, adding {C}{C}.
Repeat.

Combos with "Iron League Steed"